During drilling a hydrocarbon well, drilled cuttings are generated. Formation cavings from an unstable wellbore are also often produced when the wellbore is created. On some occasions, junk may fall into a wellbore and may have to be cleaned out or removed before further normal drilling can proceed. Large junk, such as a piece of steel pipe may have to be milled down to small pieces in order to be carried to the surface by fluid from the wellbore. These drilled cuttings, formation cavings, debris, junk, milled pieces or similar can be called unwanted solids. Apparently too much of these unwanted solids in a wellbore may reduce the drilling efficiency or hamper the drilling operations. When too much of drilled cuttings are not cleared from the bottom hole and stick to a drill bit causing bit balling, the bit may have to be pulled out of the hole for cleaning or replacing the bit.
Typically, during a conventional wellbore drilling operation and drilling fluid circulation process, drilling fluid is pumped down through a conduit such as hollow drill pipe connected to a drill bit at the far end. The drilling fluid is pumped out of the drill pipe through nozzles on the drill bit and into the annulus formed between the wellbore and the drill pipe. Driven by pump pressure, the fluid discharged out of the drill bit flows upward and carries unwanted solids to the surface. At the surface, the unwanted solids are separated from the drilling fluid by a solids control system, such as a shale shaker, and the clean drilling fluid without the unwanted solids such as cuttings and debris is pumped down the drill pipe again.
Drilling fluid has a certain capacity of carrying the unwanted solids. A higher pump rate for a higher flow rate in a wellbore can be used to improve the carrying capacity. However, the pump rate may be limited by other factors such as maximum working pressure, maximum pump strokes per minute and/or maximum available hydraulic horsepower for a pump. The pump rate may also be limited by the allowed circulation pressure on the wellbore that may fracture the wellbore when the pressure is too high. Increasing fluid viscosity may improve the carrying capacity. However, the viscosity of the drilling fluid also has a limitation. For example, too viscous a fluid can cause excessively high circulation pressure even with a low pump rate.
During completion or recompletion of an oil well, some solids such as gravels or proppants may need to be clean out from the wellbore. When a completed well with gravel pack has been used over years, it may have to be repacked. In this case, the existing gravels in the well, sands, etc. may need to be removed from the well. A well completed by frac pack with proppants may also need to be refraced. In such a case, the proppants in the wellbore have to be removed first. Sometimes the gravels or proppants may be too large and too heavy for regular fluid used to clean the wellbore at a maximum pump rate to carry them out of the wellbore, especially when the wellbore has a long lateral horizontal interval.
During drilling or completion of an oil or gas well, it is often necessary to displace one fluid out of the wellbore or to a different location. When displacing fluid in a wellbore with a different fluid, at the interface, the two fluids tend to commingle and to reduce the displacement efficiency. In some cases, the displacing fluid may bypass the fluid to be displaced causing channeling. It is preferred that the displacing fluid and displaced fluid are least commingled at the interface of the two fluids so that the fluid to be displaced can be efficiently removed. A typical example is displacing drilling mud with cement slurry or cement spacer fluid. Another example is to displace a water based drilling fluid in the wellbore with an oil based drilling fluid. Another example is displacing a drilling fluid with a completion fluid. Another example is to displace a chemical pill to a fracture intercepting a wellbore. In order to maintain the effectiveness of the pill, avoiding commingling with either the fluid ahead of the pill or the displacing fluid behind the pill may be very important.
Others have attempted to improve the carrying capacity of drilling fluid by adding fibers to the drilling fluid. However, fibers that are too short do not have much effect, and fibers that are too long tend to entangle and clog tubing and/or other flow restrictions such as a valve. A concentration of these fibers may improve the carrying capacity of the fluid. Short fibers such as 20 millimeters long may be an optimal length. To increase the carrying capacity, more fibers may be needed. However, too many fibers added to a drilling fluid can make the fluid too viscous such that it no longer behaves as a fluid and tends to clog piping and valves. Properly dispersing fibers into drilling fluid is a very difficult and time consuming process. Another problem with adding fibers to drilling fluid is that when the drilling fluid with fibers dispersed therein is carried to the shale shakers for cuttings separation, the fibers tend to “blind” the screens or plug the meshes causing the shakers to lose their solids separation function.
Due to the limitations of prior art methods, the carrying capacity of a drilling fluid is often not sufficient to transport unwanted solids from a wellbore to the surface. For example, it may be very difficult to clean the wellbore of unwanted solids, drilled cuttings and debris in drilling an extended reach well that has a long horizontal lateral wellbore of approximately 10,000 ft.
Davis, U.S. Pat. Nos. 6,016,872 and 6,164,872 disclose methods for cleaning debris from a wellbore and includes injecting hydrophilic fibers selected from the group consisting of polyolefins, polyesters and nylons, suspended or dispersed in a water based or oil based liquid into the bore and forcing the suspension through the length of the bore, to its open end. In particular, the suspension is directed through sections of the bore holding quantities of debris formed from the drilling operation. The suspension loosens the debris and sweeps substantial quantities of debris from the wellbore.
Palmer et al, U.S. Pat. No. 6,419,019 discloses an improved method for transport of particulate matter in a wellbore fluid, and particularly the transport of particulate matter in subterranean wells, such as hydrocarbon wells, by using translocating fibers and/or platelets fibers to aid in transport of the particulate matter. Additional embodiments include the removal of particulate matter (particles) and particle deposits, such as from drill cuttings, during the drilling of wells, and the removal of particulate matter deposits in cleanout operations.
Wang, U.S. Pat. No. 7,741,247 discloses methods and compositions for sealing fractures, voids, and pores of subterranean rock formations, and sealing off regions of a borehole with one or more openings, such as one or more fractures, voids, and or pores, and around a tubular string with a borehole seal such as a packer or plug. A carrying fluid is utilized to transport a filtration material into the opening to create a bridge, which at least partially seals the opening, but still provides a flow path that permits fluid flow therethrough. A solid material and/or settable material may then utilize the fluid flow subsequently or be simultaneously spotted with or behind the filtration material to thereby form compositions which effectively seals off the flow path into the one or more openings. In one embodiment, the filtration material which provides a plurality of fluid flow paths may comprise a multitude of foam rubber elements having a plurality of cells that permit fluid flow therethrough and define the plurality of fluid flow paths through the filtration material. The multitude of foam rubber elements may be elastic to compress and expand to thereby conform to any subterranean openings.